In optics, the depth of field (“DOF”) is the range between the nearest object and the furthest object in the scene that appears acceptably sharp in the image. A lens can only precisely focus on a single depth within a scene, as such sharpness gradually decreases on either side of the focus distance. Objects that fall within the depth of field are considered to have acceptable sharpness.
Some cameras have a variable focus lens while others have a fixed-focus lens. A variable focus lens enables the camera to translate its depth of field to focus on objects at variable distances from the camera. A fixed-focus camera does not have this ability and therefore has a non-translatable depth of field. Variable focus lenses are typically more bulky and more expensive than fixed-focus lenses and often include auto-focus circuitry with an actuator to move the lens system back and forth to achieve the best focus at a large range of object distances. Auto-focus introduces a number of disadvantages such as an increase in cost, size, weight, power consumption, and focus latency as the optical components are moved.
A fixed-focus lens of a fixed-focus camera typically has a depth of field biased towards the far field (e.g., greater than 1 m). This means that the fixed-focus camera typically does not have a macro-capability to focus on the near field (objects positioned close to the camera). For example, a picture taken of a book positioned at typical reading distances ends up unsatisfactorily blurry.
At least one technology exists for extending the depth of field of a fixed-focus camera and is aptly referred to as Extended Depth of Field (“EDOF”). This technology uses lenses designed to purposefully have large longitudinal chromatic aberration. FIG. 1A illustrates an example EDOF lens 100. As illustrated, the red (R), green (G), and blue (B) wavelengths are focused to different focal points by EDOF lens 100. Doing so extends the depth of field of EDOF lens 100 compared to standard lenses, which typically strive to minimize chromatic aberration to reduce image blurriness.
The mean transfer function (“MTF”) is a measure of sharpness after a ray of light passes through an optical system. An MTF of 1.0 means that the ray of light loses no sharpness after passing through the optical system. FIG. 1B illustrates a demonstrative MTF curve 101 of a standard lens system and a demonstrative MTF curve 102 of an EDOF lens system. As illustrates, the EDOF technique operates to broaden MTF curve 102 while slightly lowering the MTF within the DOF. In essence, the EDOF transfers sharpness along the object distance axis (depth axis extending from the lens system towards the object) from outside the EDOF to within the EDOF, but in so doing slightly lowers the overall MTF value.
In order to obtain an acceptably sharp image when using an EDOF lens, significant post processing steps, to convert the chromatically dispersed image into a sufficiently clear image, are required. Thus, EDOF cameras consume greater power relative to a conventional fixed-focus lens and can be more expensive due to the additional post processing circuitry. Because of the required post processing, an EDOF lens can only be used with an EDOF image sensor capable of performing the post processing functionality. When designing camera systems, an EDOF lens limits the designer's image sensor options.